use JsonNode.isNil rather than == nil to work around Nim issues (#167)

json_rpc/client.nim

@@ -38,7 +38,7 @@ method close*(client: RpcClient): Future[void] {.
   discard
 
 template `or`(a: JsonNode, b: typed): JsonNode =
-  if a == nil: b else: a
+  if a.isNil: b else: a
 
 proc processMessage*(self: RpcClient, line: string) =
   # Note: this doesn't use any transport code so doesn't need to be
@@ -163,7 +163,7 @@ proc createRpcFromSig*(clientType, rpcDecl: NimNode): NimNode =
   else:
     # native json expected so no work
     callBody.add quote do:
-      `procRes` = if `rpcResult` == nil:
+      `procRes` = if `rpcResult`.isNil:
         newJNull()
       else:
         `rpcResult`

json_rpc/jsonmarshal.nim

@@ -148,7 +148,7 @@ proc fromJson*[N, T](n: JsonNode, argName: string, result: var array[N, T]) =
 
 proc unpackArg[T](args: JsonNode, argName: string, argtype: typedesc[T]): T =
   mixin fromJson
-  if args == nil:
+  if args.isNil:
     raise (ref ValueError)(msg: argName & ": unexpected null value")
   {.gcsafe.}:
     fromJson(args, argName, result)